FR2705340A1 - Manufacture of silicon carbide foam from a silicon impregnated polyurethane foam. - Google Patents

Abstract

The invention relates to a process for preparing a silicon carbide foam comprising attacking a polyurethane foam with an alkaline solution, in impregnating it, after rinsing and drying, with a suspension of silicon powder in an organic resin, in gradually heating to polymerize the resin, carbonize the polyurethane foam and the resin, and finally carburize the silicon contained in the resin suspension with the carbon resulting from the carbonization of the foam and the resin. The foams obtained are characterized by a high macroporosity and a variable mesoporosity according to the carburizing temperature. The invention finds its application in the manufacture of catalyst supports for exhaust pipes and filters for diesel engines.

Description

MANUFACTURE OF SILICON CARBIDE FOAM FROM

  IMPREGNATED POLYURETHANE FOAM

OF RESIN CONTAINING SILICON

TECHNICAL FIELD OF THE INVENTION

  The invention relates to the technical field of structures

  porous or silicon carbide foams for use as catalysts or catalyst supports for the chemical or petrochemical industry or catalysts or filters for exhaust pipes.

PROBLEM.

  The specific surface area of a catalyst is due to three types of porosity: a macroporosity due to pores with an average diameter greater than 2 μm, a mesoporosity due to pores with a mean diameter of 30-350 angstroms and a microporosity due to pores of average diameter 5-15 angstroms. A catalyst must have sufficient macroporosity to give the gases to be treated access to the micro- and above all the mesopores responsible for the catalytic activity itself. For certain catalytic applications, in particular for the catalysis of the oxidation of exhaust gases, it is necessary to choose the geometry of the support or of the catalyst and to make it in the form of monolithic parts, while ensuring the accessibility of the catalyst to gas to be treated by its macroporosity. For Diesel engine exhaust filters, the problem is essentially the same, except that the mesoporosity does not need to be so developed since filtration involves only

  physical phenomena and not a catalytic reaction.

  The inventors have raised the problem of obtaining catalyst supports made of silicon carbide foam in monolithic form and having a high macroporosity

  open allowing easy access to the reaction gases.

OBJECT OF THE INVENTION

  The subject of the invention is a process for manufacturing catalyst supports or silicon carbide filters having the properties indicated above, from a polyurethane foam which is impregnated with a suspension of silicon in a polymerizable organic resin and which is heated to a high temperature to successively polymerize the resin, carbonize the resin and the polyurethane foam, and finally carburize the carbon from the resin and the foam. It also relates to porous silicon carbide structures that can be applied to the production of pots

  Catalysts or Filters for Diesel Engines.

PRIOR ART.

  European Patent EP-B-0313480 (Péchiney Electrometallurgie) discloses a process for producing silicon carbide grains having a specific surface area of at least 100 m 2 / g intended to serve as a catalyst support and consisting in generating SiO vapors by heating an SiO 2 + Si mixture at a temperature between 1100 and 1400 C under a pressure of between 0.1 and 1.5 hPa and reacting these vapors with reactive carbon with a specific surface area greater than 200 m2 / g at a temperature of between 1100 and

1400 C.

  The European patent application EP-A-0440569 (Péchiney Electrometallurgie) describes a process for obtaining porous solid bodies of high surface area refractory carbide, characterized in that a polymeric and / or copolymerizable organic compound, carbonizable and capable of giving a solid carbon skeleton, with a metal or metalloid powder or one of its carbon-reducible compounds, the mixture is shaped, the organic compound is cross-linked or hardened, and heat-treating the shaped mixture to carbonize between 500 and 1000 C said compound,

then to operate carbonization.

  European patent application EP-A-0511919 (Péchiney Electrometallurgie) describes a support on which the catalytically active product is deposited. The carrier has mechanical or physical properties of interest for the required operating conditions, but a poor surface area. The catalytically active product, a

  metal carbide, is obtained by immersing the support in a suspension of a reducible metal compound in a solution of an organic compound, carbonization of this compound, reduction of the metal compound and carburization of the metal. The carbide thus obtained has a high specific surface area.

  Preferably, the support consists of silicon carbide prepared by carbonization of a paste containing silicon, carbon and an organic resin. Unexamined European Patent Application 92- 420429 (GIE Pechiney Research) discloses a carbide foam. metal or silicon carbide intended to serve as catalyst or catalyst support for the chemical or petrochemical industry or for the exhaust pipes, as well as the process for producing it. The foam is in the form of a three-dimensional network of interconnected cages whose arete length is between 50 and 500 micrometers, whose BET surface is between 20 and 100 m 2 / g. The manufacturing process consists of starting from a carbon foam, increasing its specific surface area by an activation treatment with carbon dioxide, and finally putting the foam thus activated in contact with a volatile compound of the metal whose desired level is to be obtained. None of these methods describes contacting an organic suspension containing silicon with a precursor

organic foam.

DESCRIPTION OF THE FIGURES

  FIG. 1, unique, shows the distribution of the pore volume as a function of the pore diameter of a foam sample according to the invention.

DESCRIPTION OF THE INVENTION

  The process for manufacturing silicon carbide with a high specific surface area from a polyurethane foam according to the invention comprises the following steps: a) etching of the polyurethane foam with an alkaline solution, rinsing and drying; b) impregnating the polyurethane foam with a suspension of silicon powder in an organic resin whose molecule contains oxygen atoms, the weight ratio of silicon to resin being between 0.58 and 1.17; c) removing, for example by centrifugation, the excess of the suspension so that the ratio of the mass of resin impregnating the polyurethane to the mass of polyurethane is between 2.5 and 5; d) polymerization of the resin contained in the suspension by gradual increase of the temperature to 250 C with a rate of rise in temperature close to 5 C / min; e) simultaneous carbonization of the polyurethane foam and the resin by gradually increasing the temperature from 250 ° C. to 1000 ° C. with a rate of rise in temperature of about 1 ° C./min under a non-oxidizing atmosphere; f) carburizing the silicon contained in the resin suspension by the carbon resulting from the carbonization of the foam and the resin by gradually increasing the temperature of 1000 C at a temperature T of between 1300 and 1600 C with a rising speed of temperature close to 3 C / min in a non-oxidizing atmosphere and maintained for two hours at the temperature T, still under a non-oxidizing atmosphere;

  g) cooling the silicon carbide thus obtained.

  Each of these successive steps is now described in detail. A) etching the polyurethane foam with an alkaline solution, rinsing and drying; Polyurethane foam, widely used in everyday life (upholstery of seats, mattresses) is in the form of a very porous structure since its apparent density varies from 0.03 to 0.09 g / cm3 for a mass true volume of polyurethane 0.8. This porosity is very coarse, of the order of 50 pm to a few hundred of 20 pm. It is, in addition largely closed, that is to say that most pores do not communicate with each other and do not communicate with the outside. It is therefore necessary, to allow access of gases to the core of the catalyst support that is to be manufactured, to transform this closed porosity into open porosity. This is the object of the first step consisting of a gentle attack intended to dissolve the walls between the pores so as to make them communicate. It is done in an alkaline solution, preferably 4% sodium hydroxide at a temperature of 65 C for about 5 to 10 minutes. After that, the foam is rinsed with running water, dried in an oven at 150 C for half an hour or in the oven at

microwave for 15 minutes.

  b) Impregnation of the polyurethane foam with a

  suspending silicon powder in an organic resin.

  c) removing the excess of the suspension so that the ratio of the mass of resin impregnating the polyurethane to the mass of polyurethane is between 2.5 and 5; The choice of the resin is dictated by two considerations: its viscosity and the mass percentage of oxygen in the polymerized resin molecule. The viscosity must be chosen low enough so that the resin suspension can penetrate to the heart of the foam, less than 3 Pa.s. This viscosity can be adjusted by diluting the resin with a solvent such as alcohol or by adding very fine carbon black. Preliminary tests, consisting of immersing cubic samples of foam 30 mm edge and to check if the resin was successful

  to penetrate to heart, can be usefully applied.

  The mass percentage of oxygen is of great importance because at the time of carbonization of the impregnated foam, it contributes by a controlled oxidation of the carbon which is formed, to activate this carbon that is to say to develop its mesoporosity and to increase its surface area. It has been found that the mass percentage of oxygen in the polymer obtained from the resin should be at least 15% and

  preferably close to 25%. Among these resins are the polycarbonates, the polyesters and especially the polymers obtained by polycondensation of furfuryl alcohol, the percentage of oxygen of which is about 26%, which constitutes the preferred polymer.

  The diameter distribution of the silicon grains must be

  centered on an average value of less than 75 micrometers and, preferably, all the grains will have a diameter of less than 10 micrometers.

  The quantity and the composition of the impregnating suspension are defined by two coefficients: a = mass of resin / mass of polyurethane = Mw / Mpu b = mass of silicon / mass of resin = Msi / Mr In all that follows, when it comes to mass Mr

  of resin, it is necessary to understand the total resin plus the possible polymerization catalyst. For example, to the preferred resin which is furfuryl alcohol, hexamethylenetetramine is added as the catalyst.

  Regarding the coefficient a, it is clear that if it is

  too low the carbon structure obtained will be very porous and will therefore have insufficient mechanical strength; if it is too high, the macroporous structure will be blocked by the carbonization products of the resin and will not be preserved.

  However, the amount of resin must be sufficient to impregnate the polyurethane mass to the core. It may thus be necessary to adjust the viscosity of the resin hot by varying the amount of polymerization catalyst and cold viscosity by diluting it in

  alcohol or by adding carbon black.

  The inventors have found that this coefficient should be between 2.5 and 5. This limitation of the value of a can be explained as follows: The carbon resulting from the carbonization of the polyurethane and the resin occupies a certain "real" volume or "full", (quotient of the carbon mass MC by its density dc), equal to VC. To obtain sufficient solidity of the carbonaceous foam, this volume of carbon Vc must occupy at least 25% of the "real" or "full" volume Vpu of the initial polyurethane (quotient of the mass of polyurethane Mpu by its density dpu) . To preserve a macroporous structure of the carbonaceous foam, this volume of carbon Vc must occupy at most 100% of the "real" or "full" volume Vpu of the initial polyurethane. If we call x the ratio VC / Vpu which is a volume yield of the carbonization, then we must have: 0.25 <x <1 A simple calculation makes it possible to calculate the yield in weight Rx corresponding to x: Rx (in%) = 100.Mc /(Mpu.(l + a)) = 100. x. (dc / dpu) / (1 + a) (1) The coefficient b is easily deduced from the stoichiometry of the SiC formation reaction: Si + C = SiC The mass of silicon MSi = (28/12) .MC = 2,333.MC (atomic masses of Si and C = 28 and 12, respectively) Gold according to (1), Mc = (R / 100) .Mpu. (L + a) We thus have finally: MSi = 2,333. (R / 100) .Mu (1 + a) (2) and MSi = b. Mr with b = (Msi / Mpu). (Mpu / Mr) b = 2,333 (R / 100). (1 + a). (1 / a) (3) In the field of the invention, x is generally close to 0.5 and the densities of carbon and polyurethane being approximately 2 and 0.8 respectively, the formula (1) can be written: R = 100. (0,5.2 / 0,8) / (1 + a) = 125 / (1 + a) (4) By combining (3) and (4), the value of b = MSi / Mr is equal to: b = 2.333 (R / 100). (1 + a). (1 / a) = 2,917 / a (5) This last relation (5) expresses that for a mass b.Mr

  of silicon, added to a mass of resin during the preparation of the mixture, the ratio of the mass of resin to the mass of polyurethane should not exceed 2.917 / b.

  For example, for a value of b of 0.79, the maximum value

  A is 3.7 and the corresponding yield R is about 27%.

  For extreme cases: x = 0.25 and a = 2.5, b is equal to 0.58 x = 1 and a = 5, b is equal to 1.17 In practice, a silicon suspension is prepared. in the resin in the proportions defined by the coefficient b. The foam is impregnated by simply immersing it in this resin, possibly under vacuum or under pressure. It is verified that the foam is well impregnated to heart. In general the impregnation rate, found by weighing, is greater than the value

  the coefficient referred to a. The quantity of excess resin is then removed by spinning until this coefficient is obtained which is checked by weighing. This spin at 100025 rpm may take several hours.

  In order to check by weighing the quantity of impregnating agent in the impregnated foam, the ratio c of the total mass (foam + impregnation product) is used to the mass of polyurethane foam. This ratio is equal to: c = (Mpu + Mr + Msi) / Mpu = Mpu (1 + a + ba) / Mpu = 1 + a. (B + 1) (6) or by combining (5) and (6): Cmax = 3.917 + a = 3.917 + 2.917 / b The spinning will then be checked by successive weighing of the sample. It will be continued until the total mass is equal to or less than Cmax Mpu d) polymerization of the resin contained in the suspension by gradually increasing the temperature to 250 ° C. with a rate of rise in temperature close to 5 ° C. in; e) simultaneous carbonization of the polyurethane foam and the resin by gradually increasing the temperature from 250 ° C. to 1000 ° C. with a rate of rise in temperature close to C / min under a non-oxidizing atmosphere; silicon contained in the resin suspension by the carbon resulting from the carbonization of the foam and the resin by gradually increasing the temperature of 1000OC at a temperature T between 1300 and 1600 ° C with a rate of rise in temperature close to 3 C / min under a non-oxidizing atmosphere and maintained for two hours at temperature T, still under non-oxidizing atosphere; These three operations are done successively in an oven

  in an inert atmosphere containing not more than 100 ppm of oxygen, for example under argon scavenging.

  Verification that all available carbon has been converted to SiC can be done by simply weighing the carbide foam. Indeed, the mass of SiC, MSiC is related to the mass of carbon Mc by the stoichiometric relationship: MSiC = MC. (40/12) = 3,333.Mc Gold from relation (1), Rx (in fraction) = Mc / (Mpu (1 + a)) MC = Rx. Mpu. (1 + a) MSiC / Mpu = 3,333.Rx. (1 + a) Assuming that relation (4) is satisfied, we finally obtain: MSiC / Mpu = 3,333. 1.25 = 4.17 (8) This last relation makes it possible to calculate the theoretical mass of silicon carbide that must be obtained from a

  given mass of initial polyurethane.

  The choice of the final temperature T is dictated by the importance

  the volume of mesoporosity that one wants to achieve. This volume is even lower than the temperature T is higher.

  Thus, for the preparation of catalyst supports, a temperature T of between 1300.degree. C. and 1400.degree. C. is preferred, whereas for the preparation of diesel engine filters, a higher temperature T of between 1400.degree. C. and 1600.degree. C. is preferred. The product obtained is in the first case a catalyst or support of silicon carbide catalyst having a large open macroporosity allowing easy access to the reaction gases, characterized in that its BET surface area is between 25 and 50 m 2 / g, in that its porosity, bimodal, comprises a mesoporosity centered on a pore diameter value of between 0.0275 and 0.0350 μm with a standard deviation less than 0.0160 μm and a macroporosity centered on a pore diameter value of between 100 and 150 pm with a standard deviation less than 50 pm and that the

  carbide contains not more than 0.1% by weight of residual metal.

  The permeability of the carbides according to the invention was compared with that of a conventional catalyst support, in cordierite, of the same shape and of the same size. This is, in fact, a very important characteristic of use for muffler catalysts which must not cause excessive pressure drop in the high flow rate of the exhaust gases. The pressure loss per unit length P / l is given by the formula: P / 1 = pd / k1.S + m.d2 / k2.S2 where P is the pressure drop 1 the length of the sample S la sample section p the viscosity of the fluid m the density of the fluid of the fluid flow k1 and k2 two coefficients characteristic of the permeability of the sample, k1 being the so-called darcian permeability coefficient and k2 the so-called permeability coefficient non-Darcian. It should be noted that, all else being equal, the pressure drop P is all the lower, that is to say that the permeability is higher when k1 and k2 are larger.25 The table below gives the values of the coefficients k1 and

  for the classical cordierite catalyst and for the SiC foam according to the invention.

  k1 (m2) k2 (m) Catalyst cordierite 9.o10-9 1,4.10- 4 SiC foam 8.10-9 1,4,10-4 It can be seen from this table that the coefficients k1 and k2 are of the same order of magnitude for the silicon carbide according to the invention only for cordierite and that their

  permeabilities are therefore comparable.

  The silicon carbide foam can then, like cordierite, be subsequently impregnated with a platinum, rhodium or palladium active phase acting as a catalyst. The product obtained is, in the second case, a silicon carbide filter having a large open macroporosity allowing easy access to the gases, characterized in that its BET surface area is less than 5 m 2 / g, in that its porosity comprises a very high mesoporosity. low and a macroporosity centered on a pore diameter value of between 100 and 150 μm with a standard deviation of between 30 and 50 μm and that the carbide contains no more than

  0.1% by weight of residual silicon.

EXAMPLES.

Example 1

  A sample of polyurethane foam with a volume of 90 cm 3 and a bulk density of 0.03 g / cm 3 is etched with a 4% sodium hydroxide solution at 65 ° C. for 8 minutes. There is a loss of mass of 18.4%. The foam, which weighed 2.7 g at the start, now weighs only 2.20 g. A mixture by weight of 95% of furfuryl alcohol and 5% of hexamethylenetetramine as a polycondensation catalyst is prepared, but the action begins only from about 170 ° C. The mixture thus remains stable at room temperature, which allows subsequent operations without polymerization of the resin. To this mixture is added silicon powder with a mean grain diameter of 60 μm, at a rate of 7.9 g of silicon per 10 g of resin. The coefficient b is therefore equal to 0.79. The polyurethane foam is then immersed in the suspension obtained until the impregnation reaches the heart of the foam. This is followed by spinning so that the weight of resin impregnating the sample is reduced to 5.52 g. The weight of silicon is then 4.36 g and the total weight of the sample impregnated with 12.08 g. The coefficient a is 5.52 / 2.20 = 2.51 and the ratio c is 12.08 / 2.20 = 5.49, less than: Cmax = 3.917 + 2.917 / b = 7, 61 It is therefore in the presence of a lack of impregnation which will result in an amount of SiC less than the theoretical amount and the presence of an excess of carbon in the carburized foam. The impregnated foam is then heat-treated in order, successively: to polymerize the resin by increasing the temperature to 250 ° C. with a rate of rise in temperature close to 5 ° C./min; carbonizing the polyurethane foam and the resin by increasing the temperature from 250 ° C. to 1000 ° C. with a rate of rise in temperature close to 1 ° C./min under a non-oxidizing atmosphere; carburizing the silicon by the carbon resulting from the carbonization of the foam and the resin by increasing the temperature from 1000 ° C. to 1350 ° C. with a rate of rise in temperature close to 3 ° C./min under a non-oxidizing atmosphere and holding for two

  hours at 1350 C, still under non-oxidizing atmosphere. After this treatment, the foam weight obtained is 6.98 g.

  After a new treatment, in air at 800 ° C., there is a weight loss of 6.5 * corresponding to the combustion of the excess carbon, which reduces the weight of SiC foam to 6.5330 g less, '(8) after the theoretical maximum amount of

  4.17. 2.20 = 9.174 g. The amount of residual silicon is much less than 1%.

Example 2

  A sample of polyurethane foam with a volume of 43.75 cm 3 and a bulk density of 0.07 g / cm 3 is etched with a 4% sodium hydroxide solution at 65 ° C. for 8 minutes. There is no loss of mass. The sample thus weighs 3.06 g. A mixture by weight of 95% of furfuryl alcohol and 5% of hexamethylenetetramine is prepared as in Example 1. To this mixture is added powdered silicon whose average grain diameter is 60 μm, at the rate of 7.9 g of silicon per 10 g of resin, as in Example 1 and, to facilitate the impregnation, 0.15 g of carbon black, again for 10 g of resin. The coefficient b remains equal to 0.79. The polyurethane foam is then immersed in the suspension obtained until the impregnation reaches the heart of the foam. This is followed by a spin so that the weight of resin impregnating the sample is reduced to 11.74 g. The weight of silicon is then 9.28 g and the total weight of the sample impregnated with 24.08 g. The coefficient a is equal to 11.74 / 3.06 = 3.84 and the ratio c is equal to 24.08 / 3.06 = 7.86, a value slightly greater than: cmax = 3.917 + 2.917 / b = 7 Thus, we are in the presence of a slight excess of impregnation which should cause a slight excess of silicon compensated, however, by the additional addition of carbon black in the resin. In fact, we are very close to the theoretical amounts of carbon and silicon which will result in a quantity of SiC very close to the theoretical amount without excess carbon in the carburized foam. In fact, after heat treatment under the conditions of Example 1, the weight of foam obtained is 13.30 g and, according to (8), the maximum theoretical amount is 4.17. 3.06 = 12.8 g. The amount of residual silicon remains lower than

1% Example 3

Example 3

  A sample of polyurethane foam having a volume of 72 cm 3 and a bulk density of 0.067 g / cm 3 is etched with a 4% sodium hydroxide solution at 65 ° C. for 8 minutes. There is a loss of mass of 2.33%. The foam, which weighed 4.83 g initially, now weighs only 4.72 g. A mixture by weight of 95% of furfuryl alcohol and 5% of hexamethylenetetramine is prepared as in Example 1. To this mixture is added silicon powder with an average grain diameter of 60 μm, at the rate of 7.9 g of silicon per 1010 g of resin. The coefficient b is therefore equal to 0.79. The polyurethane foam is then immersed in the suspension obtained until the impregnation reaches the heart of the foam. This is followed by spinning so that the weight of resin impregnating the sample is reduced to 19.24 g. The weight of silicon is then 15.2 g and the total weight of the impregnated sample 39.16 g. The coefficient a is equal to 19.24 / 4.72 = 4.08 and the ratio c is equal to 39, 16 / 4.72 = 8.30, greater than: cmax = 3.917 + 2.917 / b = 7, 61 is thus in the presence of an excess of impregnation which will result in an amount of SiC greater than the theoretical amount. Indeed, after heat treatment under the conditions of Example 1, the weight of foam obtained is 23.1 g, higher, according to (8) than the maximum theoretical amount of 4.17. 4.72 = 19.7 g. The amount of silicon

residual remains less than 1%.

Example 4

  The porous texture of the carbide foam obtained in Example 2 was determined in comparison with that of a carbide foam obtained by reaction of carbon foam with silicon monoxide according to the teaching of the European patent application. published 92-420429 and marked "SiO". This latter foam containing a substantial amount of unreacted carbon after carburization, the comparison also relates to carbide foams having undergone a subsequent oxidation treatment for 3 hours in air at 800 C intended to remove this excess carbon. . This treatment was also applied to the foam of Example 2 although it contains little or no free carbon. He is identified

  in the table below by the word "oxidized".

  FOAM PORES METH. MAX.ABSC. MAX. DSB. DEVIATION

m2 / g Ipm cm2 / g pm ****************************************** ****************** EX 2 meso nitrogen 0,0290 0,02106 0,0139 EX 2 meso nitrogen 0,0350 0,01825 0,0156 oxidized 26 SiO meso nitrogen 0 , 0124 0.01825 0.0156 ****************************************** ****************** SiO meso nitrogen 0.0124 0.01825 0.0156 oxidized 36 EX 2 meso mercury 0.0275 0.014 0.0087 EX 2 macro mercury 117.5 0.1025 41.97 The second column indicates the nature of the pores (meso or macropores) evidenced by the method used, which is carried in the third column: nitrogen adsorption or mercury porosimeter. In the case of nitrogen adsorption, the BET surface is also given in this column. The fourth and fifth columns indicate the abscissa (in pm) and the ordinate (in cm3 / g) of the maximum in the distribution of the pore volume as a function of their diameter. Finally the sixth column

  indicates the standard deviation of the distribution.

  The conclusions drawn from this table are as follows: the carbide foams according to example 2 of the invention have a bimodal distribution whether or not they have been oxidized: a first mode is at 0.290 μm (foam unoxidized) or 0.350 μm (oxidized foam) with standard deviations of 0.139 and 0.156 μm, respectively. The oxidation treatment has the effect of widening the mean diameter of the mesopores; the carbide foams obtained from gaseous SiOs have a mesopore distribution centered on a lower neighboring value before oxidation and of the same order after oxidation; the carbide foams according to Example 2 have a second mode at 117.5 μm (macropores). In Figure 1, the pore distribution of the sample of Example 2 is shown. We can clearly see that the distribution of these

  Macropores form a very sharp peak centered on 100-150 pm.

  This is one of the essential characteristics of these foams.

  Finally, there is a good agreement between the pore distributions calculated by adsorption-desorption of nitrogen and those

  measured by mercury porosimeter.

Claims (17)

  A method of manufacturing silicon carbide foam from a polyurethane foam comprising the steps of: a) etching the polyurethane foam with an alkaline solution, rinsing and drying; b) impregnating the polyurethane foam with a suspension of silicon powder in an organic resin whose molecule contains oxygen atoms, the weight ratio of silicon to resin being between 0.58 and 1.17; c) removing, for example by centrifugation, the excess of the suspension so that the ratio of the mass of resin impregnating the polyurethane to the mass of polyurethane is between 2.5 and 5; d) polymerization of the resin contained in the suspension by gradual increase of the temperature to 250 C with a rate of rise in temperature close to 5 C / min; e) simultaneous carbonization of the polyurethane foam and the resin by gradually increasing the temperature from 250 ° C. to 1000 ° C. with a rate of rise in temperature close to 1 ° C./min under a non-oxidizing atmosphere; f) carburizing the silicon contained in the resin suspension by the carbon resulting from the carbonization of the foam and the resin by gradually increasing the temperature of 1000 C at a temperature T of between 1300 and 1600 C with a rising speed of temperature close to 3 C / min in a non-oxidizing atmosphere and holding for two hours at the   temperature T, always in a non-oxidizing atmosphere; g) cooling the silicon carbide thus obtained.   2. A method of manufacturing high surface area silicon carbide foam from a polyurethane foam according to claim 1, characterized in that   the temperature T is between 1300 and 1400 C.   3. A method of manufacturing silicon carbide foam from a polyurethane foam according to claim 1, characterized in that the temperature T is between 1400 and 1600 C.   4. Process for manufacturing silicon carbide foam   according to one of claims 1 to 3, characterized in that the attack by an alkaline solution is made with a   approximately 4% sodium hydroxide solution at 65 ° C. for 5 to 10 minutes.   5. Process for manufacturing silicon carbide foam   according to one of claims 1 to 4, characterized in that the polyurethane foam has been impregnated by vacuum immersion   in the silicon suspension in the resin.   6. Process for manufacturing silicon carbide foam   according to one of claims 1 to 4, characterized in that the polyurethane foam has been impregnated by immersion under   pressure in the silicon suspension in the resin.   7. Process for manufacturing silicon carbide foam   according to one of claims 1 to 6, characterized in that the polymer obtained from the resin contains in its   molecule at least 25% by weight of oxygen atoms.   8. Process for manufacturing silicon carbide foam   according to claim 7, characterized in that the resin contained in the suspension is a furfuryl resin.   9. A method of manufacturing silicon carbide foam according to one of claims 1 to 8, characterized in that the   silicon contained in the suspension has a particle size centered on a mean value of less than 75 micrometers.   10. A method of manufacturing silicon carbide foam according to claim 9, characterized in that the silicon contained in the suspension has a particle size less than 10 micrometers. 11. A method of manufacturing silicon carbide foam according to claim 1, characterized in that the silicon carbide is finally subjected to an oxidation treatment to remove a possible excess of carbon. 12. Process for producing a silicon carbide support catalyst with a large specific surface area according to the   Claim 2, characterized in that the silicon carbide is then impregnated with an active phase based on platinum, rhodium or palladium.   13. Silicon carbide intended in particular to serve as catalyst or catalyst support and having a large open macroporosity allowing easy access to the reaction gases, characterized in that its BET surface area is between 25 and 50 m 2 / g, in that its bimodal porosity comprises a mesoporosity centered on a pore diameter value of between 0.0275 and 0.0350 μm with a standard deviation of less than 0.0160 μm and a macroporosity centered on a pore diameter value between 100 and 150 μm with a standard deviation less than 50 μm, in that the carbide   contains no more than 0.1% by weight of residual metal.   14. Silicon carbide intended in particular to serve as catalyst or catalyst support according to claim 13, characterized in that the coefficient k1 of its areal permeability is between 4 and 12. 10-9m2 and the coefficient k2 of its permeability non-Darcian is between 1.1 and 1.7. 10-4m.35 15. Silicon carbide intended in particular for the manufacture of exhaust gas filters for diesel engines, characterized in that its BET surface area is less than 5 m 2 / g, in that its porosity comprises a very low mesoporosity and a centered macroporosity. on a value of   pore diameters between 100 and 150 μm with a difference   type between 30 and 50 pm and in that the carbide contains not more than 0.1% by weight of residual silicon.   16. Application of silicon carbide according to one of claims 13 or 14 for the manufacture of catalytic converters.   17. Application of silicon carbide according to claim   in the manufacture of exhaust filters for diesel engines.